Field of the Invention
Physical solvents such as methanol, NMP (normal methyl pyrolidinone) [U.S. Pat. No. 3,103,411 and U.S. Pat. No. 4,208,382]selexol, propylene carbonate [U.S. Pat. No. 2,926,751]and others are widely used for the removal of H.sub.2 S from gases such as natural gas and syngas from coal gasification. In such cases the solvent is contacted counter currently with the gas and the solvent is then heated and later stripped to remove one of the dissolved gases, H.sub.2 S [U.S. Pat. No. 4,198,388]. In many of those applications, CO.sub.2 is also present and it is important to minimize the CO.sub.2 that is released together with the H.sub.2 S in the stripping of the solvent. This is due to the fact that high concentration of CO.sub.2 would make complete conversion of the H.sub.2 S in a chemical process more difficult. This complexes the design of the extraction column, and there are a number of commercial designs that have solved that problem [Kohl and Riesenfeld, 1985, Newman, 1985]. The details of these designs are not relevant to our inventions. It is enough to know that satisfactory design exists.
One important and well known feature of these designs is that they require a solvent which is highly selective for H.sub.2 S as compared to CO.sub.2. This is especially important if the concentration of CO.sub.2 is much higher than that of H.sub.2 S.
It is obvious to those skilled in the art that any improvement in selectivity has great value in making the cost of separating and extending the range of relative conditions over which the method is commercially viable. The field of the present invention is the description of a method which simplifies the search for improved solvents as well as specific examples of solvent mixtures with greatly enhanced selectivity properties.
Partially Miscible Solvent Mixtures With A Critical Point of Miscibility.
Let us consider a binary mixture of solvents which have a critical point of miscibility. The meaning of such a critical point can be explored by a phase diagram (see FIG. 1).
This figure describes the concentration of component A and B as a function of temperature. The region marked "one phase" represents conditions of complete miscibility of components A and B, whereas regions marked "two phase" correspond to separation into two phases. When the phase diagram shows a maximum (FIG. 1a), it is called an upper critical solution temperature (UCST), whereas a minimum (FIG. 1b) corresponds to a lower critical solution temperature (LCST).
Liquids posses also a vapor-liquid critical point. Above the critical temperature, there is only one fluid phase independent of pressure. In the last twenty years, it was found that near this critical point there are some very interesting thermodynamic properties, [Paulaitis, et al., 1983]. Those unique properties have resulted in the development of separation processes named supercritical extraction. However, that field should not be confused with the present art.